1. Field of the Invention
The present invention relates to an improved pyrometer which is utilizable for the precise measurement of temperatures of materials heated at a temperature higher than 1000.degree. C., such as a molten metal or glass and to a method of fusing a terminal end of an alumina pipe of high purity used, for example, for a pyrometer.
2. Description of the Prior Art
Prior art protective pipes for a platinum or platinum-rhodium wire used for an ordinary pyrometer are predominantly alumina having a purity of at most about 99.7%. In general, the protective pipe is fused on one terminal end to form a closed-end pipe, like a test tube, into which a temperature-measuring resistance element comprising the platinum or platinum-rhodium wire is inserted. Such a conventional alumina protective pipe is formed using silica or the like as a binder. On heating such a pipe at a temperature above 800.degree. C., therefore, a gas is evolved from impurities contained in the alumina and/or the binder, which contaminates the platinum or platinum-rhodium wire to deteriorate its temperature characteristics. If alumina has a purity as high as 99.9% or more, no contaminating gas will be evolved therefrom even on heating it at a temperature above 800.degree. C. On the other hand, when a protective pipe for the temperature-measuring element is to be manufactured from such highly pure alumina, a pipe (tube) having both terminal ends open can easily be manufactured but a pipe having one terminal end closed can hardly be obtained because of lack of any binder incorporated into the alumina. In the event a binder is incorporated into such highly pure alumina, the pipe manufactured therefrom tends to bend at a high temperature and may be broken on rapid heating or cooling. Furthermore, external air intrudes into the pipe creating the possibility that the temperature-measuring element will become contaminated with impurities contained in the air. Even if this problem is overcome by carrying out the process under vacuum, there is a detrimental drawback in that a pipe of a uniform quality with either terminal end closed by fusing cannot be obtained with good reproducibility.
Under the circumstances, there is a great demand to develop a new type pyrometer which overcomes all the drawbacks in the prior art pyrometers by using a highly pure alumina protective pipe free from the fear of contamination.